2. • Organisms such as whales, seals, and manatees
are only a few that make up the category of
aquatic mammals. Aquatic mammals are
organisms that live in and rely on aquatic
ecosystems for a part of or for all of their lives.
These aquatic ecosystems include both marine
and freshwater systems. Aquatic mammals are
warm-blooded animals that breathe air through
their lungs, have had hair at some point of their
development, and nurse their young with their
milk.
3. • Aquatic mammals have entered aquatic ecosystems 7 times
throughout history. Out of these seven groups, only five are
still alive today. These groupings formed from mammals
that originally lived on land, then adapted and evolved to
live in the water. One of the first groups to originate was
the Order Cetacea. This group’s closest relative is the
hippopotamus and it includes dolphins, porpoises, and
whales. They moved to water about 50 million years ago
during a period of time called the Eocene. The Order
Sirenia also originated during the Eocene. This group
includes dugongs and manatees, and their closest ancestors
are elephants. The third group that was known to adapt
and evolve to inhabit aquatic ecosystems was the Suborder
Pinnipedia. They moved to the aquatic ecosystem during
the Oligocene period around 30 million years ago. This
group is thought to have evolved from bears and is related
to bears seen today.
5. • Adaptations for Aquatic Environments
• Aquatic mammals possess diverse adaptations
specific to their environments and lifestyles.
Some of these adaptations can be seen in their
body form. Aquatic mammals tend to have a
streamlined shape with no protruding
appendages. This increases their efficiency when
moving through their ecosystem, specifically in
areas of high viscosity. Their limbs evolved into
fins and flippers; these are used for balance,
steering and directional control, and propulsion
and speed. Perhaps the most
6. • Well-known adaptation is the layer of insulating
blubber aquatic mammals possess. The blubber helps
to keep them warm and to improve their buoyancy in
the water.
• Aquatic mammals also have the advantage of an
adapted circulatory system. These organisms have
evolved a blood chemistry that allows them to retain
more oxygen. This, in turn, enables them to remain
underwater for longer periods of time. They are also
able to send blood to their vital organs directly.
• These organisms use echolocation to communicate.
This method of communication helps them avoid
obstacles in the water and aids them in detecting
potential prey. Additionally, it allows them to
communicate over hundreds, sometimes thousands, of
miles underwater in their ecosystem.
7. • In aquatic mammals, the forelimbs are transformed
into skin-covered, un-jointed paddles or flippers,
having no separate indication of fingers. These paddles
or flippers can move as a whole only at the shoulder
joint. The broad and flattened paddles or flippers serve
as balancers and provide stability during swimming.
• In aquatic mammals, the nostrils are placed far back on
the top of head so that animal can breathe air without
raising head much out of water. The nostrils can also be
closed by valves during diving under water.
• In aquatic mammals, during lactation, ducts of
mammary glands dilate to form large reservoirs of
milk, which is pumped directly into mouth of young by
the action of special compressor muscle. This
arrangement facilitates suckling of young under water.
8. • In aquatic mammals, oblique diaphragm makes
the thoracic cavity larger dorsal and barrel-
shaped for providing more space to lungs for
expansion.
• In aquatic mammals, large unlobulated and highly
elastic lungs ensure taking down maximum air
Lore submergence. Like swim bladders of fishes,
the dorsal lungs also serve as hydrostatic organs
in maintaining a horizontal posture during
swimming.
• In aquatic mammals, elongated, tubular and
intra-narial epiglottis, when embraced by the soft
palate, provides a continuous and separate air-
passage, thus, allowing breathing and feeding
simultaneously.
9. Loss of structures
• There is a loss of hairs. Skin surface usually remains
smooth and glistening due to loss of hairs except for a
few sensory bristles on snout or lips in some cases.
• Pinnae are also absent. Presence of hairs and pinnae
may obstruct or impede the ever flow of water over
body surface and interfere with the speed and
elegance of movement through water.
• Nictitating membranes, eye cleansing glands, lacrimal
glands and all kinds of skin glands (sweat and
sebaceous) are also absent because they would have
been useless under water.
• Fingernails are absent except for traces in foetus.
• Scrotal sacs are also absent and testes remain inside
abdomen.
10. Development of New Structures
• Tail Flukes:
• In aquatic mammals, some large, lateral or horizontal
expansions of the skin develop on tail. These
expansions are called tail flukes. These are not
supported by fin-ray. Their up and down strokes not
only propel the body through water but enable rapid
return to the surface for breathing after prolonged
submersion.
• Dorsal Fin:
• In most Cetacea develop an unpaired adipose dorsal fin
without internal skeletal support. It serves as a rudder
or keel during swimming.
11. • Blubber:
• In aquatic mammals, the blubber is the thick subcutaneous
layer of fat, which compensates for the lack of hairy
covering. Blubber acts as a heat insulator. It not only retains
the warmth of the body but also provides a ready reservoir
of food and water during emergency.
• The fat also reduces the specific gravity of the animal, thus,
imparting buoyancy. Blubber also provides an elastic
covering to allow changes in body volume during deep
diving and also counteracts the hydrostatic pressure.
• Baleen:
• In whalebone whales, teeth are absent. Instead, the upper
jaw carries two transverse rows of numerous triangular
fringed horny plates of baleen or whalebone. These serve
as an effective sieve for straining plankton (mostly kril)
which forms their chief food.
12. • Foam:
• Foam is a fine emulsion of fat, mucus and gas. Each middle ear
cavity sends an inner pneumatic prolongation, which meets with
the fellow on the other side below the skull. These extensions
contain foam. It probably serves to insulate sound and improves
audition or hearing under water.
• Melon:
• In some aquatic mammals, the melon is a receptor present in front
of nostrils It consists of a fatty mass traversed by muscle fibres. It
possibly serves to detect pressure changes in water.
• Harderian Glands:
• In aquatic mammals, eyes under water remain protected by a
special fatty secretion of Harderian glands.
•
Ref: Vertebrates by Jordan Verma
• Notes on Zoology
